An electron accelerator intended preferably for use in medical radiotherapy is known as described in U.S. Pat. No. 4,121,109. In this electron accelerator a target is exposed to the electron beam issuing from the beam exit window of the acceleration tube. In beam direction beyond the target an electron absorber is arranged, through which the electrons remaining in the X radiation are filtered out. In beam direction beyond the electron absorber is a collimator for masking the roentgen ray field maximally being used. A compensation body made of low atomic number metal such as iron is secured on the collimator centered on the masking aperture thereof and extending into the body thereof. The compensation equalizes the radiation intensity over the total width of the roentgen ray field. In such an electron accelerator it is found to be disadvantageous that the low-energy X-ray component is relatively high.
To reduce the low-energy X-ray component, previously known apparatus uses a deflecting magnet which deflects the electron beam by 270.degree. and focuses the electrons of a given energy. In this way the target is hit only be electrons of the selected acceleration energy. Such a deflecting magnet, however, is extremely expensive to construct and also requires considerable space between the beam exit window of the acceleration tube and the target. This, in turn, adversely affects the overall size of the accelerator.